The present invention relates to an electric power tool operated by a motor.
A conventional motor-operated electric power tool uses a semiconductor switch for closing/interrupting a current path from a power source to a motor. In the electric power tool constituted as such, the semiconductor switch, provided on the current path from the power source to the motor, is controlled to be turned ON/OFF in accordance with a state of a contact switch operated by a user of the tool. Thus, a driving current (high current) for driving the motor does not directly flow to the contact switch. The contact switch to be used can be the one that has small contact capacity.
In a constitution such that the current path from the power source to the motor is closed/interrupted by the semiconductor switch, however, the motor continuously operates when short-circuit fault (which keeps the semiconductor switch normally-ON) occurs to the semiconductor switch. Therefore, application of a semiconductor switch to an electric power tool is restricted to a very limited use in practice, on the assumption that short-circuit fault may occur to the semiconductor switch.
A method to deal with such fault of a semiconductor switch is disclosed in Unexamined Patent Publication No. 57-109230 as an example. In this method, parallel use of a semiconductor switch and a contact switch is proposed. Specifically, on the current path from the power source to the motor, not only a triac as a semiconductor switch but also a contact power switch is provided which is turned ON (becomes conductive)/OFF (becomes nonconductive) directly by user operation.